1. Field of the Invention
The present invention relates to a process for producing a magnetic recording medium and, in particular, a high density magnetic recording medium that has a low level of head wear as well as excellent transport durability.
2. Description of the Related Art
In recent years, there has been a trend for the density of magnetic recording to increase and for the recording wavelength to decrease. There is a high demand for magnetic recording media with a magnetic layer having a smoother surface, thereby reducing spacing loss. As one method for smoothing the surface of the magnetic layer, there is a technique involving increasing the degree of dispersion of a magnetic liquid. In general media dispersion, the degree of dispersion can be increased by dispersing for a long time using, as media beads, high density media such as zirconia beads, employing a high packing ratio of the media beads, a high circumferential velocity, etc. In cases where the surface of the magnetic layer can be made smoother by the above-mentioned technique, fine abrasive particles are localized on the very surface of the magnetic layer, the degree of packing of magnetic material on the very surface decreases, thereby causing the problem of low output, and the protrusion height of the abrasive on the surface of the magnetic layer decreases, thereby causing the problem of poor transport durability. When an abrasive that is used for smoothing the surface of the magnetic layer is made into fine particles, the above-mentioned problems are further worsened.
As one means for solving these problems, JP-A-7-14159 (JP-A denotes a Japanese unexamined patent application publication) discloses a method in which an abrasive and carbon black used in a magnetic liquid are each made into a paste separately from a magnetic coating solution comprising a ferromagnetic powder and a binder, they are dispersed individually, and then mixed. In this method, the magnetic coating solution, an abrasive liquid paste, and a carbon black liquid paste are dispersed individually to their own optimally dispersed states, and then mixed to prepare a coating solution. The problem of the degree of packing of the magnetic layer and the problem of poor durability due to a lower protrusion height on the surface of the magnetic layer can thus be substantially solved. However, this method for preparing a magnetic coating solution by separate dispersion and mixing can cause new problems such as an increase in head wear due to aggregation of the abrasive and a decrease in the durability due to aggregation of the abrasive and the carbon black.
As hereinbefore described, it is difficult to produce a magnetic recording medium having a magnetic layer with a smoother surface and giving a high output, excellent transport durability, and low head wear, and there is a desire for two or three of these characteristics to be achieved simultaneously.
As a result of an intensive investigation by the present inventors in order to simultaneously satisfy two or three of the above-mentioned characteristics, a specific process for producing a magnetic recording medium has been found.
It is an object of the present invention to provide a process for producing a magnetic recording medium having a smooth magnetic layer surface, good electromagnetic conversion characteristics, excellent transport durability, and low head wear.
As a result of an intensive investigation by the present inventors into a process for producing a magnetic recording medium in order to achieve the object, a magnetic recording medium having good electromagnetic conversion characteristics, excellent transport durability, and low head wear has been produced.
The object can be attained by a process for producing a magnetic recording medium in which a magnetic layer comprising a ferromagnetic powder and a binder is provided on a non-magnetic flexible support, the process comprising a step (1) for separately preparing a magnetic liquid comprising mainly a ferromagnetic powder and a binder, and an abrasive liquid paste comprising an abrasive and a binder, and then a step (2) for subjecting the abrasive liquid paste to an ultrasonic dispersion treatment and then mixing it with the magnetic liquid to prepare a magnetic coating solution for the magnetic layer.
The object can also be attained by a process for the production of a magnetic recording medium in which a magnetic coating solution comprising a ferromagnetic powder and a binder is coated on a non-magnetic flexible support, the process comprising a step (1) for separately preparing a magnetic liquid comprising mainly a magnetic powder and a binder, an abrasive liquid paste comprising an abrasive and a binder, and a carbon black liquid paste comprising carbon black and a binder, and then a step (2) for subjecting the abrasive liquid paste and the carbon black liquid paste individually to an ultrasonic dispersion treatment and then mixing them with the magnetic liquid to prepare a magnetic coating solution.